This invention relates to an HF alkylation process and the efficient recovery of hydrocarbon product.
In the HF alkylation process, hydrocarbon products can be produced by alkylation reactions involving the combination of isoparaffins having low boiling temperatures such as isobutane with olefins having low boiling temperatures such as ethylene, propylene, butene, pentene and the like in the presence of HF acid catalyst. In the typical HF alkylation process, hydrocarbon feed primarily comprising C3 to C6 olefins with limited quantities of propane and lighter compounds are mixed wtih a butane mixture of isobutane and n-butane. This hydrocarbon mixture is reacted in the presence of HF acid catalyst to form a hydrocarbon mixture of high octane alkylate along with unreacted isoparaffins, normal paraffins, light hydrocarbons, including propane and ethane, and HF acid. The reaction product mixture must subsequently undergo a phase separation of the HF acid and the hydrocarbon with the hydrocarbon phase further undergoing fractionation to separately recover the products of alkylate, propane and normal butane.
The amount of propane produced in an HF alkylation process can vary substantially depending upon the process feed composition and the alkylation reactions which take place. In particular, there are two primary factors which determine the amount of propane recovered in the HF alkylation process. The first factor being the amount of propane actually charged to the unit which can vary upwardly to twenty percent or more of the hydrocarbon feed. The other factor having a bearing upon the amount of propane produced is related to the quantity of the olefin propylene introduced into the process unit and the extent to which the hydrogen transfer reaction takes place. In the hydrogen transfer reaction, as much as twenty percent of the propylene can be converted to propane by reacting propylene with HF acid to form the hydrogen saturated compound of propane. In many alkylation processes, as much as thirty percent of the olefin charge can be propylene.
The potentially wide variability of propane processed in an alkylation unit can pose operating problems in the various pieces of fractionation equipment. In a common process, the separated hydrocarbon alkylation product is charged to a fractionation section where alkylate, unreacted paraffin, isoparaffins, and propane and lighter hydrocarbons are recovered. Typically, the overhead of a fractionation column comprising essentially propane and lighter hydrocarbons and contaminating quantities of HF acid is charged to a propane stripping column where a portion of the propane and essentially all the light hydrocarbons and HF acid is driven overhead, and the remaining portion of propane and essentially all heavier hydrocarbons are recovered as a bottoms product. The bottoms product is essentially free of HF acid, but requires further treatment to remove any remaining traces of HF acid, fluoride compounds, and other contaminants prior to storage and sales. Many operating problems occur in the propane stripper in which an essentially on-specification propane product is produced.
In designing an alkylation process, the propane stripper must be designed and sized to accommodate the situations of maximum propane production, or at least, it should be designed to handle the typical case of propane production. But, occasionally during operation there are dramatic changes in feedstock composition resulting in substantial reductions in the quantity of propane produced and processed in the alkylation unit. This reduction in propane production consequently results in a large reduction in the charge rate to the propane stripper, which has earlier been designed for a much greater charge rate. The reduced charge rate has a negative effect upon the operation of the propane stripper by making it more difficult to control. Because of the difficulty in the control of the HF stripper at lower charges rates, off-specification propane product is often made resulting in the further contamination of products and in the transfer of HF acid to other propane treatment vessels where additional hazards are created.